Protein Trafficking And Degradation Flashcards
Ribosomes are directed to bind to ER when involved in synthesizing products destined for ____
. Insertion into cell membranes
. Function w/in lysosomes
. Secretion outside cell
Free ribosomes synthesize proteins for what locations?
. Nucleus
. Mitochondria
. Peroxisomes
Signal sequences
. Structural features found w/in sequences of protein being produced
. Recognized by organelles
. Direct protein o appropriate location for further modification to make protein functional
Proteins synthesized on bound ribosomes move ___
Outside cell
Proteins synthesized on free ribosomes are located ____
W/in the same cytosol
N-terminal hydrophobic signal sequence (NH2-terminal signal/leader sequence)
. Presence on newly synthesized protein causes ribosome synthesizing it to bind to ER
. N-terminal: amino terminal region of protein
. Hydrophobic: contains AA that don’t interact w/ water
. Recognized by signal recognition particles (SRPs)
Signal recognition particles
. Complex made of proteins and RNA
. facilitate attachment of ribosome to ER
. SRP bound to leader sequence bind to a SRP receptor on membrane of ER
. Once ribosome is docked to ER membrane, the nascent polypeptide enters into ER lumen
N-glycosylation
Once in ER, leader sequence is cleaved by proteases
. Remainder of AA sequences enters space of lumen
. most proteins w/ Asn-X-Ser or Asn-X-Thr have 14 sugar, branched oligosaccharide added to it from membrane lipid dolichol
. Co-translational
Transitional element
. Area of smooth ER where proteins go through a series of membrane spaces w/in ER
. Membrane of transitional element surrounds and encloses nascent polypeptide until it buds off into a transport vesicle
. Vesicle fuses w/ Golgi
Vesicles from smooth ER fuse with what part of Golgi?
Cis-golgi
Processes w/in Golgi
. Glycosylation: addition of carb
. Sulfation: addition of sulfur
. Phosphorylation: addition of phosphate
. Proteolysis: cleavage of peptide bonds
Mannose-6-phosphate tag
. Occurs on proteins that will function in lysosome
. Phosphorylated on mannoses added to the protein
Trans golgi network
. Last sorting and packaging region of golgi
. Polypeptides sent to lysosomes or out of cell
Lysosome proteins and characteristics
. Acidic internal pH
. Degrade unwanted macromolecules
. Contains hydrolases that degrade unwanted macromolecules at acidic pH
. Lysosomal membrane keeps hydrolases compartmentalized and sequestered from cytoplasmic macromolecules
How trafficking of acid hydrolases works
. Addition of mannose-6-phosphate to precursors
. Segregation of those precursors in trans golgi by binding to mannose-6P receptors clustered in clathrin-coated pits
. Formation and using of clathrin-coated vesicles from TGN
. Fusion of transport vesicle w/ endolysosomes and loss of clathrate coat
. Dissociation of hydrolases from mannose-6P receptors because of acid pH of endolysosome
. Removal of phosphate via phosphatase
. Recycle receptors
I cell disease
. Problem of targeting all acid hydrolases to lysosomes
Types of lysosomal storage diseases
. Hunter and hurler
. Tay Sach’s
. Gaucher
Pathway of proteins destined for secretion outside o the cell from the TGN
. Bud off from TGN encapsulated in transport vesicle
. Fuses w/ plasma membrane (constitutive) or stored in cytoplasm until release (regulated)
Default pathway for proteins synthesized on free ribosomes
. Remain in cytosol
. Structural features can be incorporated to direct these proteins to specific organelles
Cytosolic proteins
. Intracellular proteins that function outside boundaries of organelles
Nuclear proteins
. Contain nuclear localization signal (NLS) that allows them through nuclear pore
. Binds strongly to importin (receptor on nuclear envelope)
. Involved in DNA replication or RNA Synthesis
Mitochondrial proteins
. Contain N-terminal mitochondrial import sequence
. Nuclear proteins synthesized on free ribosomes associate w/ chaperone proteins to keep them unfolded prior to entry in mitochondria
. TOM and TIM have proteins cross through outer and inner mitochondrial membranes and into the matrix
Peroxisomal proteins
. Hydrolytic enzymes
. Contain C-terminal tripeptide that functions as peroxisomal targeting signal
. ER-derived
. Breakdown fatty acids and purines (AMP/GMP)
. Detox hydrogen peroxide
. Cholesterol and bile synthesis in hepatocytes
. Synthesis of myelin
What are some inherited diseases caused by peroxisomal dysfunction?
. X-linked adrenoleukodystrophy
. Zellweger syndrome
Insulin biosynthesis
. Translation of mRNA gives preproinsulin w/ signal peptide that directs the protein into ER lumen
. Proteolytically cleaved to yield proinsulin inside the ER
. Proinsulin transported to golgi and proteolytically cleaved into insulin and C-peptide
. Insulin stored in cytosol in granules or vesicles
. Insulin released via regulated exocytosis
Protein has a finite life span is eventually degraded in order to _____
. Supply AA for new protein synthesis
. Remove transcription factors, enzymes or any other protein whose activity isn’t needed
. Prevent accumulation of abnormal proteins
Pathways to degrade proteins
. Lysosomal
. Proteasome
Lysosomal degradation pathway
. Degrades membrane and extracellular proteins via endocytosis
. Degrades proteins w/ extended half-lives
. Occurs via autophagy (autophagosome vesicles engulf small amounts of material from cytoplasm or organelles using fragments of ER membrane)
. Vesicles fuse w/ lysosomes triggering release of hydrolytic enzymes and degradation proteins
Proteasome pathway
. Degrades intracellular proteins w/ short half-lives
. ATP dependent
. Get ubiquinated, once 4 ubiquitin in chain make it a target for proteasome
. Proteins recognized and degraded by 60-protein complex (26S proteasome)
. Proteasome ha central core (20S particle) of 7 beta-subunits and 2 outer rings (19S particle) w/ 7-alpha subunits
. 19S particles recognize proteins, remove ubiquitin, unfold, and translocate protein to core to be hydrolyzed into peptides
. Peptides exit proteasome and are degraded in cytosol
Process of ubiquitination of proteins
. Ubiquitin: 76 AA protein
. Ubiquitin-activating enzyme (E1) bind ubiquitin and transfers it to ubiquitin-conjugating enzyme (E2)
. Ubiquitin ligase enzyme (E3) promotes transfer of ubiquitin from E2 to Lys residues of protein recognized by E3 to be degraded
Proteins w/ long half life
. N-terminal Met, Ser, Ala, Thr, Val, or Gly: half life of 20 hours
Proteins w/ half life less than 3 minutes
. Phe . Leu . Asp . Lys . Arg
Proteins that are rapidly degraded
. Proteins rich in Pro, Glu, Ser, and Thr (PEST)
Bortezomib
. Blocks proteolytic action of proteasome
. Used to treat patients w/ multiple myeloma
. Inhibits degradation of several proteins involved in cell cycle control and apoptosis
